Conventionally, there is widely used a linear motion apparatus such as a linear guide and a ball screw. In this kind of linear motion apparatus, a linear motor or a servo motor is used to drive a movable body, and a position, speed, and acceleration of the movable body is controlled by changing a current supplied from a motor driver (motor control device).
When a foreign substance falls in a traveling path of the movable body, or when chips or dust invades into a ball circuit of the linear motion apparatus, the smooth movement of the movable body is interrupted, which causes a malfunction or a failure. Therefore, usually an overload alarm function is provided in the motor driver, and a warning is issued when the supply current exceeds a predetermined alarm value (threshold).